Problems with replacing a static id with variable

Question

I have a script that loads info from external XML file and inserts it into the HTML page on click. Here's the code of this script:

  var xmlhttp;
function loadXMLDoc(url,target) {
xmlhttp = null;
if (window.XMLHttpRequest) { // code for all new browsers
    xmlhttp = new XMLHttpRequest();
}
else if (window.ActiveXObject) { // code for IE5 and IE6
    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
if (xmlhttp != null) {
    xmlhttp.onreadystatechange = state_Change;
    xmlhttp.open("GET", url, true);
    xmlhttp.send(null);
}
else {
    alert("Your browser does not support XMLHTTP.");
}
}
function state_Change() {
if (xmlhttp.readyState == 4) { // 4 = "loaded"
    if (xmlhttp.status == 200) { // 200 = OK    
        var markers = xmlhttp.responseXML.documentElement.getElementsByTagName("marker");
        for (var i = 0; i < markers.length; i++) {
            var name = markers[i].getElementsByTagName("name")[0].firstChild.nodeValue;
            //etc...
document.getElementById(target).innerHTML += '<li>' + name + '</li>\n';
        }
    }
    else {
        alert("Problem retrieving XML data");
    }
}
}

Here's the HTML:

  <ul id="list_puncts">
<li><a href="javascript:;" onclick="loadXMLDoc('./content/geo_points/slovenia.xml','list_sl')">Republika Slovenija (RS)</a>
 <ul id="list_sl">
   <!--here should be some info from XML file-->
     </ul></li>
     <li><a href="javascript:;" onclick="loadXMLDoc('./content/geo_points/horvatia.xml','list_hr')">Republika Hrvatska (RH)</a>
    <ul id="list_hr">
    <!--here should be some info from XML file-->
    </ul></li>
   </ul>

However, it does not work - after the link is clicked, XML gets loaded (it can be seen in Firebug), but the second variable - target - cannot make its way into the state_Change function, so no real action is done. If target in document.getElementById(target).innerHTML is replaced by some static id (like list_sl), it is working, but I have many of these links in the HTML, not only Slovenia and Horvatia, so the variable is strongly needed.

Thanks for any help.

Answer 1

Put the state_Change logic in an anonymous function within loadXMLDoc.

function loadXMLDoc(url,target) {
    // ... code to instantiate XMLHttprequest goes here ...

    if (xmlhttp != null) {
        xmlhttp.onreadystatechange = function() {
            // Your state_Change() logic goes here.
            // This has access to the variable "target" because
            // it's within the same enclosing function's scope.
        };
        xmlhttp.open("GET", url, true);
        xmlhttp.send(null);
    }
}

The drawback to this is that a new instance of the function is instantiated each time loadXMLDoc is called.

Answer 2

If you use global variables (both for XMLHttpRequests and your targets), you can't support parallel requests (i.e. a user clicks on another link before one XMLHttpRequest returns). One way to implement it is to keep the xmlhttp variable in your loadXMLDoc function's scope and add your target as a property to the XMLHttpRequest object:

function loadXMLDoc(url, target) {
    var xmlhttp;
    // create XMLHttpRequest instance
    if(xmlhttp != null) {
     xmlhttp.onreadystatechange = state_Change;
     xmlhttp.targetListId = target;
     xmlhttp.open(); // etc.
    }
};

In your state_Change function you can access the XMLHttpRequest instance using the event's target property:

function state_Change(e) {
    var xmlhttp = e.target;
    // check readystate & status

    var targetList = document.getElementById(xmlhttp.targetListId);
    targetList.innerHTML = ''; // fill with data, etc.
};